Zoom lens systems have been used in a variety of applications, such as image capture devices for capturing still images or moving images. Examples of still images may include static pictures of landscapes, wildlife, or sports. Examples of moving images may include motion pictures of movies, film, and video. Although zoom lens techniques are known for all of these various applications, differences in these applications may lead to differences in the development and structure of zoom lens systems for different applications. In other words, one zoom lens system may be more suitable than another for a certain application.
Focus Breathing
By way of example, considerations that may be significant for capturing moving images may be negligible or non-existent for capturing still images. One such consideration may be the effect of focus breathing. When the focus of a lens system is adjusted, lens elements for focusing may move. This movement may result in a change in the total focal length of the lens system. As total focal length may be related to zooming, the field of view (or angle of view) may change, similar to a zooming effect. For instance, as one changes the focus back and forth between the foreground to the background, the field of view may change such that it appears to be “breathing” (or zooming in and out) during the focus adjustment in real-time. Although these changes in the field of view may actually result from adjustments in focus settings, the changes may appear to be results from adjustments in zoom settings, even when zoom settings have not been adjusted.
In an example from a cinematography application, such as a movie scene, one may want to change focus from one actor to another actor during a conversation in the same field of view without changing the field of view, i.e., without focus breathing. Significant changes in the field of view during multiple focus adjustments may be undesirably distracting to a viewing audience.
When capturing a particular still image, one may be concerned about using a particular field of view for that particular still image capture, not about maintaining the same particular field of view for the next still image capture. With respect to this particular still image capture, the next still image capture may be completely unrelated in field of view. In other words, unlike capturing moving images, capturing still images may generally involve little or no concern about maintaining the same field of view for different still image captures with different focus settings. Furthermore, focus breathing may be detected during focus changes in real-time capturing of motion pictures, but effects during real-time focus changes may be ignored or even often unnoticed when capturing still images. Accordingly, capturing still images may generally involve little or no concern related to focus breathing.
Even when capturing moving images, focus breathing may still be a minor or negligible concern in some applications. For instance, a user of an ordinary video camcorder may be satisfied with an image capture device having a simpler optical lens system that does not include such precise controls for field of view and focusing.
Lens Shade
In addition to focus breathing, cinematography applications may also involve other considerations. For example, in order to provide high-quality image capturing for professional-level motion pictures, it is generally desirable to remove or reduce unwanted effects on the captured image. Lighting may be a crucial variable to control, and collecting light from unintended light sources may lead to some generally undesirable effects, such as ghost images.
In order to limit entrance of light into a camera lens system from unintended sources, such as unwanted sunlight, a lens shade (e.g., a matte box) may be attached to the outer lens barrel at the front of a lens system in a cinematography application. If the lens shade is too short, unwanted light may enter the lens system. If the lens shade is too long, too much light may be blocked, and the lens shade itself may enter the field of view. A lens system with a fixed front lens element may lead to fewer or no adjustments of the lens shade size and/or position. A lens system with a moving front lens element may require many adjustments of the lens shade size and/or position in order to compensate for the different positions of the moving front lens element. Therefore, for applications in cinematography, it may be highly advantageous to employ a lens system with a fixed front lens element that is stationary during functions that may involve moving lens elements, such as zoom and focus. Conversely, it may be uncommon to practice applications in cinematography with a lens system having a moving front lens element.
Lens Speed
For cinematography applications, lens speed may be another common consideration. Lens speed can be correlated to the maximum aperture of a lens, which can be quantified in terms of an F-number F/#. Aperture size and F-number are inversely related, so the maximum aperture would correspond to a minimum F-number. Also, a lens with a larger aperture would have a smaller F-number, and vice versa. For example, lens A having a larger maximum aperture (smaller minimum F-number) will be able to pass through more light to the image capturing film (or sensor) than lens B having a smaller maximum aperture (larger minimum F-number). Lens A would enable a faster shutter speed than lens B. Therefore, lens A (with a smaller minimum F-number) would be “faster” than lens B (with a larger minimum F-number). In other words, a larger aperture leads to a “faster” lens.
A common concern in cinematography applications is shooting pictures under lower illumination environments. For such environments in dim lighting, higher lens speeds are generally preferred. For example, a lens of relatively high speed may have an F-number of 2.8 or smaller.
In some applications, it may be desirable to attain image captures with specific areas that are out-of-focus, or bokeh. Bokeh can occur in an image area that is outside the depth of field. Faster lenses can have a shallower depth of field, which can be useful for providing images with bokeh. Thus, a high-speed lens may be required to attain images with a desirable amount of bokeh.
A similar parameter used in cinematography is T-number T/#. T-number is like F-number but additionally adjusted for the amount of light transmitted through the lens in actual usage. For instance, at a given lens aperture, the T-number will equal the F-number if the lens has 100% transmission, i.e., no loss of light. However, as light passes through a lens, there is loss (e.g., through absorption by the lens). Therefore, the T-number will be larger than the F-number. For cinematography applications, a minimum T-number smaller than 2.8 may be preferred. The use of T-number is relatively uncommon outside of cinematography.
Prior Art Zoom Lenses
Although zoom lens systems are known for many various applications, not all zoom lens systems are applicable for all these various applications. For example, a given zoom lens system may be particularly designed for a certain application, but not suitable for another application. Additionally, combining techniques of different zoom lens systems may involve complicated considerations and may not be simple to realize.
U.S. Pat. No. 4,815,829 to Yamanashi et al. demonstrates a telephoto zoom lens system. However, the zoom function of this system operates by moving the front lens element, i.e., the lens element at the front end of the system. In view of the lens shade consideration above with respect to a moving front lens element, this system may not be preferred for use in cinematography applications with a lens shade. Furthermore, the zoom lens examples of Yamanashi et al. have F-numbers of F/3.5 or greater, which are much slower than the relatively higher speed zoom lenses of cinematography applications, such as those with F-numbers of F/2.8 or less. In contrast, U.S. Pat. No. 7,123,421 to Moskovich et al. discloses a zoom lens system for cinematography with an F-number of F/2.7, a zoom lens with relatively high speed.
U.S. Pat. No. 4,991,942 to Fujibayashi et al. discloses a zoom lens with a first lens group that is stationary during zooming. However, this first lens group moves during focusing. Such lens group movement during focusing may contribute to a focus breathing effect. Even though this zoom lens system may be used in a video camera, there is no discussion of any technique to address the effect of focus breathing. Additionally, as the front lens element may be a moving lens element, this system may be not recommended for use in cinematography applications with a lens shade.
U.S. Pat. No. 3,598,476 to Merigold exemplifies a zoom lens with a stationary front lens element that is part of a lens group for focusing. Merigold's zoom lens functions with lens groups that move according to a particular movement plan during zooming. In contrast, the zoom lens system of Fujibayashi et al. functions with lens groups that move according to a different movement plan during zooming. This is not a trivial difference.
In the field of zoom lens systems, it is generally understood that a functional system is a complex combination of many interrelated variables (e.g., optical power, lens position, lens movement, lens size, lens thickness, lens material, number of lens elements, lens surface shaping). Changes in one of the variables generally alter the functioning of the original system (e.g., zoom operation). In order to maintain a system that functions appropriately (e.g., according to the principles of the original zoom design), changes in one of the variables generally lead to compensating changes in one or more of the other variables.
Accordingly, experimentation with any variables to incorporate the teachings of a first zoom lens system into a second zoom lens system could lead to other unintended adverse effects in the second zoom lens system. Such adverse effects may result in changing the fundamental operation of the second zoom lens system.
U.S. Pat. No. 5,717,527 to Shibayama teaches a zoom lens system with three lens groups. This zoom lens system appears to be directed to macro photography, or very close-up photography. In an embodiment, the front lens element may be stationary during zooming, but movable during focusing. However, there is no discussion of any technique to address the effect of focus breathing. Macro photography often involves capturing still images, so there may be little or no concern for focus breathing.
Also, in macro photography, the distance from the lens to the object is often very small, and the image on the image capture medium (e.g., film or digital sensor) is similar in size to the object being photographed. A zoom lens, as in Shibayama, may be designed to be optimized for such object distances and sizes. Thus, it may be unsuitable or not optimized for cinematography applications, which generally involve objects that are much farther away from a lens (e.g., ˜1 m or greater) or that are much larger in size.
Furthermore, the zoom lens examples of Shibayama have F-numbers that are F/4.0 or greater, which are much slower than the relatively higher speed zoom lenses of cinematography applications, such as those with F-numbers of F/2.8 or less. In contrast, U.S. Pat. No. 7,123,421 to Moskovich et al. discloses a zoom lens system for cinematography with an F-number of F/2.7, a zoom lens with relatively high speed.
An article, “A complete set of cinematographic zoom lenses and their fundamental design considerations,” by Zuegge et al. provides teachings for zoom lenses for cinematography applications. Zuegge et al. mentions considerations of focus breathing, a fixed overall length in view of a compendium hood, and high-speed zoom lenses. However, the zoom lenses of Zuegge et al. are specific designs that do not cover all solutions for addressing these considerations.